This invention relates generally to alloys, and more particularly to alloys formed on the surface of stainless steel.
Electrochemical conversion cells, commonly referred to as fuel cells, produce electrical energy by processing reactants, for example, through the oxidation and reduction of hydrogen and oxygen. A typical polymer electrolyte fuel cell comprises a polymer membrane (e.g., a proton exchange membrane (PEM)) with electrode layers (e.g., containing at a minimum one catalyst type and one ionomer type) on both sides. The catalyst coated PEM is positioned between a pair of gas diffusion media layers, and a cathode plate and an anode plate are placed outside the gas diffusion media layers. The components are compressed to form the fuel cell.
FIG. 1 shows one embodiment of a fuel cell 10. The fuel cell includes a PEM 15 between a pair of electrodes 20. The electrodes 20 form a cathode and an anode for the fuel cell. The electrodes 20 may be deposited onto the PEM 15, as in the CCM design, to form an MEA 25. There is a gas diffusion media (GDM) 30 adjacent to each of the electrodes 20. Alternatively, the electrodes 20 can be deposited onto the GDM, as in the CCDM design. Adjacent to each of the GDM is a fuel cell plate 35. These fuel cell plates can be unipolar or bipolar plates, as known in the art.
The anode and cathode plates are typically made of stainless steel. The surface composition of stainless steel is known to affect its properties. For example, iron contributes significantly to the composition of the passive oxide films on entry grade stainless steels and other highly alloyed stainless. In addition, the presence of iron in the passive films of nickel/chromium alloys increases contact resistance with carbon-type papers (such as are used as GDM). Chromium contributes to the corrosion resistance of stainless steel. Chromium is also a well known adhesion promoter, and it can contribute significantly to the adhesion of gaskets on stainless steels.
As a result, the surface of the stainless steel fuel cell plates has been coated to obtain the desired properties, such as corrosion resistance and adhesion. For example, plating techniques and physical vapor deposition (PVD) have been used to coat stainless steel with alloys with high chromium and/or titanium content or layers of chromium and/or titanium in order to improve the corrosion resistance, or adhesion, for example. However, these plating and PVD coating methods are expensive. Moreover, there can be adhesion problems between the additional layers.